The present invention is directed to methods of making an article bearing a relief image using a removable film that may be reused to make additional articles bearing the relief image.
Many methods of forming a relief image are known in the graphic arts. Photosensitive articles comprising an ablatable mask layer on the surface of a photosensitive polymer (or a so-called “integral mask”) may be made into articles bearing a relief image without the use of a photographic negative or other separate masking device. These photosensitive articles are formed into relief images by first imagewise exposing the photosensitive article with laser radiation (generally from an infrared laser under computer control) to selectively remove the mask layer in the exposed areas, and then overall exposing with an actinic radiation to cure the photosensitive layer in the unmasked areas. The remaining areas of the mask layer and the non-hardened portions of the photosensitive layer are then removed by one or more liquid development processes. Examples of flexographic articles having an integral mask are described in U.S. Pat. No. 5,262,275 to Fan, U.S. Pat. No. 5,705,310 to Van Zoeren, U.S. Pat. No. 5,719,009 to Fan, U.S. Pat. No. 6,020,108 to Goffing, et al., and U.S. Pat. No. 6,037,102 to Loerzer, et al. U.S. Pat. No. 6,759,175 to Daems et al. reports a method of laminating an ablatable mask layer to a UV-sensitive material with the use of an adhesive to create an integral mask on the UV-sensitive material before imaging the integral mask and curing the UV-sensitive material.
While elements having a laser ablatable mask layer allow direct imagewise exposure with a laser and do not require a separate masking device, the imaging time to create the mask is very long since the sensitivity to infrared radiation is low for the known integral mask systems. Sensitivity is generally not lower than about 1 J/cm2, and is more typically about 3 j/cm2.
In recent years attempts have been made, such as reported in U.S. Pat. No. 6,521,390 to Leinenbach, et al., to improve the infrared sensitivity of an ablatable mask layer by using heat-combustible polymeric binders and specific aliphatic diesters. Although higher sensitivity and, as such, shorter exposure time may be achieved, this construction suffers from undesirable adhesion of the ablatable mask layer to a coversheet that must be removed before exposure; see U.S. Pat. No. 6,599,679 to Philipp, et al. at C1 and C2, Table 2.
Higher sensitivity is difficult with the integral mask construction as the laser ablatable layer must satisfy a number of widely varying quality criteria; see U.S. Pat. No. 6,599,679, col. 2, line 1-29. The use of a polyether-polyurethane binder in an ablatable layer is reported in U.S. Pat. No. 6,599,679, but the enhancement in imaging speed was modest (Examples 1-3 reported at Table 2; cf. Comparative Example C6).
Furthermore, the integral mask approach for the production of flexographic printing plates requires the use of high-powered laser-equipped imagers specifically configured for imaging the integral mask flexographic articles, such as CYREL Digital Imager (CDI SPARK) manufactured by Esko-Graphics (Kennesaw, Ga.), and ThermoFlex by Creo (Burnaby, British Columbia). Because of the need for varying the thicknesses of flexographic plates depending upon the specific printing application, more than one imager may be required with the integral mask flexographic articles. In contrast, conventional imaging apparatus for “computer-to-plate” lithographic applications (e.g., TRENDSETTER from Creo), and digital proofing applications (e.g., DESERTCAT 88 from ECRM) may be used in the present invention.
An analog method is also known to produce a relief image on an imageable material. In the typical analog method, a non-adhesive mask is formed via one of many processes, such as silver halide emulsion. The mask is then placed on the photosensitive material for exposure. Since these masks do not adhere to the photosensitive material, they tend to slip during the exposure step. To keep the mask from slipping while the photosensitive material is exposed to radiation, the step of exposing is done in a vacuum. The vacuum pulls the mask to the photosensitive material thus eliminating any space or air pockets that may be trapped between the mask and the photosensitive material, thus providing optical contact between the mask and the photosensitive material. If optical contact is not achieved, the light in the exposure step may scatter when it reaches the pockets of air resulting in a relief image that is not an exact representation of the intended image on the mask. The vacuum also prevents the mask from slipping during the exposure step. This feature of analog printing methods is commonly known as “vacuum draw-down.” In at least one respect, the method of the present invention differs from the analog method in that the present invention utilizes an imaged film that adheres to the imageable article during the curing step so that the vacuum draw-down step is not required. However, the imaged film also has the characteristic in that it may be removed from the imageable article and reused.